In current enhancement of aspiring to environment-conscious manufacturing techniques, regenerable and inexhaustible energy sources are becoming more important. Photovoltaic generation using solar light, i.e., clean energy is environment-compatible, and has been extensively studied not only for basic researches but also for practical-application researches.
Photovoltaic generation converts solar light (visible light) into electrical energy. Conversion efficiency is one of indicators of the performance of a photovoltaic material, while it is a significant challenge to achieve high conversion efficiency for low costs and practical applications of solar cells. Single-crystalline silicon, polycrystalline silicon and amorphous silicon are currently regarded as a comparatively suitable material as the photovoltaic material. The eminently suitable materials include a chalcopyrite compound semiconductor with high conversion efficiency.
The chalcopyrite compound semiconductor is a direct-transition type semiconductor, generally having higher optical absorption properties than indirect-transition type semiconductors. It has been reported that the chalcopyrite compound semiconductor showed conversion efficiency up to 19% (Prog. Photov. Res. Appl. 13 (2005) 209), thus allowing it to regard the chalcopyrite compound semiconductor as a potentially suitable material for solar cells.
It is possible to provide the chalcopyrite compound semiconductor with a wide variety of band gaps. It is known that the band gap of the chalcopyrite type compound semiconductor typified by Cu(In, Ga)Se2 can be controlled by the combination of the constituent elements thereof, thereby enabling it to form an energy gap of about 1.4 eV corresponding to the average energy of visible light. This allows it to obtain higher conversion efficiency of the solar cell employing the chalcopyrite type compound semiconductor.
The chalcopyrite compound semiconductors with different energy gaps may be laminated to form a tandem cell, thereby allowing it to absorb light with different wave lengths, the enhancement of the conversion efficiency being thus expected.
When the conversion efficiency is reviewed for the typical materials such as Si currently used for a single solar cell, the single crystal Si, polycrystalline Si and amorphous Si have conversion efficiencies of 25%, 20% and 13%, respectively. III-V compound semiconductors such as GaAs show a conversion efficiency of 31.5%, and II-VI compound semiconductors such as CdS show 16%, as reported. Thus, a high conversion-efficiency means about 30% at the highest.
III-V compound semiconductors have a comparatively high conversion-efficiency, while the upper limit of the conversion efficiency thereof has been evaluated theoretically for such a single solar cell with a single gap. The theoretical evaluation reports that the upper limit is 40.7% under an ideal condition (Phys. Rev. Lett. 78, 5014 (1997), J. Appl. Phys. 32, 510 (1961)), suggesting that the conversion efficiency for the single solar cell may be enhanced just by controlling the energy gap to reach up to about 40% at the highest. Therefore, in order to acquire the conversion efficiency exceeding the theoretical value, a semiconductor device and a designing method thereof, being less conventional, are desired.